Metal cans, such as steel cans and aluminum cans, used as packaging containers for food and drinks, are being used in large quantities. To impart corrosion resistance and printability to these metal cans, solvent-based paints consisting mainly of thermosetting resins have been applied to can surfaces.
However, such application of paint lowers productivity and causes problems including environmental pollution. Therefore, in recent years, laminates have been used more frequently which are produced by laminating biaxially stretched plastic films or heat-sealable layers with such plastic film used as a substrate to provide a laminated film which is then laminated to a metal sheet and subjected to various forming and processing techniques for use as a metal can.
Metal cans covered with plastic films are produced by laminating a plastic film to a metal sheet (including those which are plated or otherwise surface-treated), such as a steel sheet or aluminum sheet, and subjecting it to forming process. Plastic films put to such use are required to have the following various properties at once:
(i) Being superior in laminatability with respect to metal sheets. PA0 (ii) Being superior in can formability. That is, the film suffers no delamination, no cracking or no pinhole formation during the can forming process. PA0 (iii) No possibility of destroying the flavor of the contents of the can (when the plastic film is used on the inner surface of the can). PA0 (iv) Retort treatment should not be attended with occurrence of water spot or white powder. (By water spot is meant a phenomenon in which a film which has been melted and non-crystallized during laminating catches water drops during the retort treatment to become crystallized and whitened. When water spot occurs, this detracts from the aesthetic feature of the goods. By white powder is meant a form of a low molecular weight substance, such as oligomer, deposited on the film surface. When a laminated film is used on the inner surface of a can, the white powder detracts from the flavor of the can contents, while when it is used on the outer can surface, the aesthetic feature of the can is spoiled by the white powder.) PA0 (1) The tensile strength is not less than 10 kgf/mm.sup.2, preferably 13 kgf/mm.sup.2. PA0 (2) The longitudinal (MD) and transverse (TD) heat shrinkage factors of the film at 150.degree. C., 30 minutes are not more than 6.0%, preferably 0.5-5.0%. PA0 (3) The optical density is not less than 0.3, preferably 0.4-0.7. PA0 (4) The whiteness is not less than 81.0, preferably 85.0.
Usually, cans are externally printed, at which time white paint is applied as an undercoat for the purpose of hiding the metal color and then printing is applied thereon. In recent years, in view of simplification of manufacturing processes (energy reduction and cost reduction) and measures against environmental problems (non-use of solvent), production of cans having laminated thereto a white film in hidability has been in progress.
As such white film, use is made of a polyester resin mixed with a high concentration of titanium oxide. Such material, however, is insufficient in whiteness and hidability and it has been desired to increase the amount of titanium. Increase in the amount of titanium, however, presents problems including the stiffening of the film surface, the wearing of dies made of metal for producing cans, and the sticking of scraped metal or titanium oxide to the film surface which results in unevenness of printing during printing and in aggravation of laminatability with respect to steel sheets.
As for such white film for metal lamination, one is disclosed in Japanese Patent Laid Open No. 5-170942 which is produced by mixing titanium oxide to copolymer polyester in order to improve can formability. Further, another is disclosed in Japanese Patent Laid Open No. 5-339391 which is produced by mixing rutile-based titanium oxide of 95% or more purity to copolymerized polyester. Further, still another is disclosed in Japanese Patent Laid Open No. 6-271686 which is produced by mixing master chips of titanium oxide of high concentration and diluting polymer having a wide viscosity size distribution in order to improve can processability and shock resistance. Further, another is disclosed in Japanese Patent Laid Open No. 6-49234 which is produced by mixing a diluting polymer of high viscosity with master chips of titanium oxide in order to improve shock resistance. Further, a laminated polyester film produced by laminating two copolymerized polyesters different in pigment concentration is disclosed in Japanese Patent Laid Open Nos. 6-39980 and 7-52351.
Although single-layer or double-layer films having titanium oxide filled into polyester resin have been proposed as described above, none of them satisfy the two requirements at the same time; improved whiteness and reduced wear of can producing dies.
Further, in recent years, in addition to the desire of further improving the whiteness and hidability of films for cans, there has been a desire that the thickness of films be further reduced for cost reduction. To meet this desire, it is necessary to incorporate titanium oxide in a higher filling density.
However, incorporation of titanium oxide in a higher filling density involves various problems; (a) during production of films, particularly during stretching, breaks are liable to occur, detracting from operatability, (b) laminatability with respect to metal sheets is aggravated, (c) during the forming process of film-laminated metal sheets, dies made of metal or the like tend to be damaged, (d) it is difficult to print onto laminate film surfaces(defects are liable to occur).